9,016 research outputs found
Modification of primordial ices by cosmic rays as simulated by cyclotron irradiation
Frozen CH4 and CH4/Ar mixtures closed into metal cuvettes and open to the vacuum were irradiated at 15 and 77 K with 10 - 20 MeV p and He-3(2+) ions in order to simulate the effect of cosmic rays on solid organic matter in space. Ices exposed to vacuum represent surfaces of icy systems whereas closed systems stand for bulk ices. The products were analyzed by MS, SEM, RBS, ERDA, H-1-NMR, HPLC, GC-MS, NEXAFS, and FT-IR. Volatile products consisted of a mixture of low molecular species, e.g., C2H2, C2H4, C2H6, and long linear aliphatic and olefinic compounds. The formation of polycyclic aromatic hydrocarbons (PAH's) and related species in solid CH4 is due to a multi-center reaction within one collision cascade and is governed by energy density effects with critical linear energy transfer values L(sub T) between 2 and 10 keV/micron. Open ices exhibit preferential hydrogen release resulting in an increased carbonization as compared to more hydrogen rich molecules protected inside large icy bodies
Global stability analysis of birhythmicity in a self-sustained oscillator
We analyze global stability properties of birhythmicity in a self-sustained
system with random excitations. The model is a multi-limit cycles variation of
the van der Pol oscillatorintroduced to analyze enzymatic substrate reactions
in brain waves. We show that the two frequencies are strongly influenced by the
nonlinear coefficients and . With a random excitation, such as
a Gaussian white noise, the attractor's global stability is measured by the
mean escape time from one limit-cycle. An effective activation energy
barrier is obtained by the slope of the linear part of the variation of the
escape time versus the inverse noise-intensity 1/D. We find that the
trapping barriers of the two frequencies can be very different, thus leaving
the system on the same attractor for an overwhelming time. However, we also
find that the system is nearly symmetric in a narrow range of the parameters.Comment: 17 pages, 8 figures, to appear on Choas, 201
Flows on scales of 150 Mpc?
We investigate the reality of large-scale streaming on scales of up to 150
Mpc using the peculiar motions of galaxies in three directions. New R-band CCD
photometry and spectroscopy for elliptical galaxies is used. The Fundamental
Plane distance indicator is calibrated using the Coma cluster and an
inhomogeneous Malmquist bias correction is applied. A linear bulk-flow model is
fitted to the peculiar velocities in the sample regions and the results do not
reflect the bulk flow observed by Lauer and Postman (LP). Accounting for the
difference in geometry between the galaxy distribution in the three regions and
the LP clustersconfirms the disagreement; assuming a low-density CDM power
spectrum, we find that the observed bulk flow of the galaxies in our sample
excludes the LP bulk flow at the 99.8% confidence level.Comment: 16 pages, 1 figur
Weak Lensing by High-Redshift Clusters of Galaxies - I: Cluster Mass Reconstruction
We present the results of a weak lensing survey of six high-redshift (z >
0.5), X-ray selected clusters of galaxies. We have obtained ultra-deep R-band
images of each cluster with the Keck Telescope, and have measured a weak
lensing signal from each cluster. From the background galaxy ellipticities we
create two-dimensional maps of the surface mass density of each cluster. We
find that the substructure seen in the mass reconstructions typically agree
well with substructure in both the cluster galaxy distributions and X-ray
images of the clusters. We also measure the one-dimensional radial profiles of
the lensing signals and fit these with both isothermal spheres and "universal"
CDM profiles. We find that the more massive clusters are less compact and not
as well fit by isothermal spheres as the less massive clusters, possibly
indicating that they are still in the process of collapse.Comment: 43 pages, 15 figures, uses aastex, submitted to ApJ 4 color plates
produced here as jpg's, larger versions of the jpgs can be found at
http://www.mpa-garching.mpg.de/~clow
Connection between accretion disk and superluminal radio jets and the role of radio plateau state in GRS 1915+105
We investigate the association between the accretion disk during radio
plateau state and the following superluminal relativistic radio jets with peak
intensity varies from 200 mJy to 1000 mJy observed over a period of five years
and present the evidences of direct accretion disc-jet connection in
microquasar GRS 1915+105. We have analysed RXTE PCA/HEXTE X-ray data and have
found that the accretion rate, , as inferred from the X-ray
flux, is very high during the radio plateaux. We suggest that the accretion
disk during the radio plateaux always associated with radiation-driven wind
which is manifested in the form of enhanced absorption column density for X-ray
and the depleted IR emission. We find that the wind density increases with the
accretion disk luminosity during the radio plateaux. The wind density is
similar to the density of the warm absorber proposed in extragalactic AGNs and
Quasars. We suggest a simple model for the origin of superluminal relativistic
jets. Finally, We discuss the implications of this work for galactic
microquasars and the extragalactic AGNs and Quasars.Comment: 9 pages, 6 Figures, Accepted for publication in Ap
Electromagnetic vortex lines riding atop null solutions of the Maxwell equations
New method of introducing vortex lines of the electromagnetic field is
outlined. The vortex lines arise when a complex Riemann-Silberstein vector
is multiplied by a complex scalar function
. Such a multiplication may lead to new solutions of the Maxwell
equations only when the electromagnetic field is null, i.e. when both
relativistic invariants vanish. In general, zeroes of the function give
rise to electromagnetic vortices. The description of these vortices benefits
from the ideas of Penrose, Robinson and Trautman developed in general
relativity.Comment: NATO Workshop on Singular Optics 2003 To appear in Journal of Optics
Maxwell Fields and Shear-Free Null Geodesic Congruences
We study and report on the class of vacuum Maxwell fields in Minkowski space
that possess a non-degenerate, diverging, principle null vector field (null
eigenvector field of the Maxwell tensor) that is tangent to a shear-free null
geodesics congruence. These congruences can be either surface forming (the
tangent vectors proportional to gradients) or not, i.e., the twisting
congruences. In the non-twisting case, the associated Maxwell fields are
precisely the Lienard-Wiechert fields, i.e., those Maxwell fields arising from
an electric monopole moving on an arbitrary worldline. The null geodesic
congruence is given by the generators of the light-cones with apex on the
world-line. The twisting case is much richer, more interesting and far more
complicated. In a twisting subcase, where our main interests lie, it can be
given the following strange interpretation. If we allow the real Minkowski
space to be complexified so that the real Minkowski coordinates x^a take
complex values, i.e., x^a => z^a=x^a+iy^a with complex metric g=eta_abdz^adz^b,
the real vacuum Maxwell equations can be extended into the complex and
rewritten as curlW =iWdot, divW with W =E+iB. This subcase of Maxwell fields
can then be extended into the complex so as to have as source, a complex
analytic world-line, i.e., to now become complex Lienard-Wiechart fields. When
viewed as real fields on the real Minkowski space, z^a=x^a, they possess a real
principle null vector that is shear-free but twisting and diverging. The twist
is a measure of how far the complex world-line is from the real 'slice'. Most
Maxwell fields in this subcase are asymptotically flat with a time-varying set
of electric and magnetic moments, all depending on the complex displacements
and the complex velocities.Comment: 3
Formation of Nitriles in the Interstellar Medium via Reactions of Cyano Radicals, CN(X 2Σ+), with Unsaturated Hydrocarbons
Crossed molecular beam experiments of cyano radicals, CN(X 2Σ+, ν = 0), in their electronic and vibrational ground state reacting with unsaturated hydrocarbons acetylene, C2H2(X 1Σ), ethylene, C2H4(X 1Ag), methylacetylene, CH3CCH(X 1A1), allene, H2CCCH2(X 1A1), dimethylacetylene, CH3CCCH3(X 1A1'), and benzene, C6H6 (X 1A1g), were performed at relative collision energies between 13.3 and 36.4 kJ mol-1 to unravel the formation of unsaturated nitriles in the outflows of late-type AGB carbon stars and molecular clouds. In all reactions, the CN radical was found to attack the π electron density of the hydrocarbon molecule with the radical center located at the carbon atom; the formation of an initial addition complex is a prevalent pathway on all the involved potential energy surfaces. A subsequent carbon-hydrogen bond rupture yields the nitriles cyanoacetylene, HCCCN (X 1Σ+), vinylcyanide, C2H3CN (X 1A'), 1-methylcyanoacetylene, CH3CCCN (X 1A1), cyanoallene, H2CCCH(CN) (X 1A'), 3-methylcyanoacetylene, HCCCH2CN(X 1A'), 1,1-cyanomethylallene, H2CCC(CN)(CH3) (X 1A'), and cyanobenzene, C6H5CN (X 1A1). In case of acetylene and ethylene, a second reaction channel involves a [1, 2]-H atom shift in the initial HCCHCN and H2CCH2CN collision complexes prior to a hydrogen atom release to form cyanoacetylene, HCCCN (X 1Σ+), and vinylcyanide, C2H3CN (X 1A'). Since all these radical-neutral reactions show no entrance barriers, have exit barriers well below the energy of the reactant molecules, and are exothermic, the explicit identification of this CN versus H atom exchange pathway under single collision conditions makes this reaction class a compelling candidate to synthesize unsaturated nitriles in interstellar environments holding temperatures as low as 10 K. This general concept makes it even feasible to predict the formation of nitriles once the corresponding unsaturated hydrocarbons are identified in the interstellar medium. Here HCCCN, C2H3CN, and CH3CCCN have been already observed; since CH3CCH is the common precursor to H2CCCH(CN)/CH3CCCN and the latter isomer has been assigned unambiguously toward TMC-1 and OMC-1, H2CCCH(CN) is strongly expected to be present in both clouds as well. The formation of isonitrile isomers was not observed in our experiments. Since all reactions to HCCNC, C2H3NC, CH3CCNC, H2CCCH(NC), H2CCC(NC)(CH3), and C6H5NC are either endothermic or the exit barrier is well above the energy of the reactants, neutral-neutral reactions of cyano radicals with closed shell unsaturated hydrocarbons cannot synthesize isonitriles in cold molecular clouds. However, in outflow of carbon stars, the enhanced translational energy of both reactants close to the photosphere of the central star can compensate this endothermicity, and isonitriles might be formed in these hotter environments as well
Observations of Rapid Disk-Jet Interaction in the Microquasar GRS 1915+105
We present evidence that ~ 30 minute episodes of jet formation in the
Galactic microquasar GRS 1915+105 may sometimes entirely be a superposition of
smaller, faster phenomena. We base this conclusion on simultaneous X-ray and
infrared observations in July 2002, using the Rossi X-ray Timing Explorer and
the Palomar 5 meter telescope. On two nights, we observed quasi-periodic
infrared flares from GRS 1915+105, each accompanied by a set of fast
oscillations in the X-ray light curve (indicating an interaction between the
jet and accretion disk). In contrast to similar observations in 1997, we find
that the duration of each X-ray cycle matches the duration of its accompanying
infrared flare, and we observed one instance in which an isolated X-ray
oscillation occurred at the same time as a faint infrared "subflare" (of
duration ~ 150 seconds) superimposed on one of the main flares. From these
data, we are able to conclude that each X-ray oscillation had an associated
faint infrared flare and that these flares blend together to form, and entirely
comprise, the ~ 30 minute events we observed. Part of the infrared emission in
1997 also appears to be due to superimposed small flares, but it was
overshadowed by infrared-bright ejections associated with the appearance of a
sharp "trigger" spike in each X-ray cycle that were not present in 2002. We
also study the evolution of the X-ray spectrum and find significant differences
in the high energy power law component, which was strongly variable in 1997 but
not in 2002. Taken together, these observations reveal the diversity of ways in
which the accretion disk and jet in black hole systems are capable of
interacting and solidify the importance of the trigger spike for large
ejections to occur on ~ 30 minute timescales in GRS 1915+105.Comment: 17 pages, 9 figures; accepted for publication in The Astrophysical
Journa
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